[!743] [test][mpi] run mpipacktest only if MPI was found

Merge branch 'run-mpipacktest-only-with-mpi' into 'master'

ref:core/dune-common @gruenich can we merge this to 2.7?

See merge request [!743]

  [!743]: gitlab.dune-project.org/core/dune-common/merge_requests/743

dune/common/parallel/test/CMakeLists.txt

@@ -42,4 +42,5 @@ dune_add_test(SOURCES mpipacktest.cc
               LINK_LIBRARIES dunecommon
               MPI_RANKS 2
               TIMEOUT 300
+              CMAKE_GUARD MPI_FOUND
               LABELS quick)